Physiological sensors are often used in medical applications to help doctors diagnose, monitor, and treat patients. Some physiological sensors use spectroscopy to provide valuable information about the patient's body tissue. Spectroscopy generally refers to the dispersion of light as it travels through a medium. A physiological sensor employing near-infrared spectroscopy may be used to detect characteristics of various body tissues by transmitting and receiving near-infrared light through the body tissue, and outputting a signal to a controller that provides valuable information about the body tissue. A doctor may use this information to diagnose, monitor, and treat the patient.
To measure the intensity of the light that travels inside the tissue, the near-infrared spectroscopy sensor may use one or more large area photodiodes mounted onto a flexible circuit board within a sensor pad. Because the photodiodes have a high equivalent resistance of the p-n junction, the sensor is very sensitive to the electromagnetic interference from other devices, such as electrosurgical equipment, electrocardiogram devices, or power supplies from medical or other electronic devices. One way to reduce the sensitivity of the near-infrared spectroscopy sensor to these other devices includes enclosing the photodiodes in a Faraday shield made from a copper mesh or plastic film covered by a transparent conductive material, such as iridium oxide. However, the Faraday shield is expensive, decreases the sensitivity of the photodiodes to the near-infrared light generated by the sensor, and reduces the flexibility of the sensor.
Accordingly, a sensor is needed that reduces or eliminates the effects of electronic devices without the added expense and/or decreased sensitivity in sensors employing the Faraday shield solution.